The present invention relates to a device for detecting a substrate such as a semiconductor wafer, intended to be used in a semiconductor device fabrication system.
In a typical process for fabricating semiconductor devices, for the purpose of introducing wafers into treatment sections such as heat treatment chambers, a container carrying untreated wafers (which in general is called a "carrier" or a "cassette") is commonly conveyed by way of an inlet/outlet port into an intermediate chamber, where the wafers are taken out from the carrier by means of a conveying mechanism such as arms for the conveyance into one of the treatment chambers. The carrier has an open bottom as known, and includes on each side thereof comb-teeth-like guide grooves extending in parallel to correspondingly guide and receive the lateral edges of the wafers so as to accommodate, for example, twenty-five wafers.
In order to control the conveying action of the conveying mechanism or to perform a control within the treatment chamber, detection must be made of the presence or absence of the wafers resting between the opposite guide grooves in the carrier placed at the inlet/outlet port. To this end, a wafer counter is provided.
Due to the opacity of silicon wafers which are generally in use as semiconductor wafers, there has been hitherto used a wafer counter of a transmission type having a light emitting section comprised of a light emitting diode and a light receiving section comprised of a photo-transistor arranged respectively on one side and the other side of respective regions where the wafers are disposed, the presence or absence of the wafers being detected based on light reception signals derived from the light receiving section.
However, there have been lately manufactured transparent quartz wafers consisting of LCD substrates in array. Such transparent wafer substantially transmits therethrough light sent out from the light emitting section, and hence the conventional transmission type wafer counter cannot detect whether a transparent wafer is present or absent. This is due to the following fact. In the presence of the transparent wafer, the amount of light received by the light receiving section slightly reduces since a small amount of light is absorbed by the transparent wafer. Nevertheless, the light receiving section receives the light from the light emitting section irrespective of the presence or absence of the wafer. The range of output voltage levels corresponding to various light receiving states has a certain width (due to, for example, unevenness in characteristics of the photo-transistors, or influence of the disturbance light), and there is only a slight difference in light receiving amount between the "presence" and "absence". It is therefore very difficult and virtually impossible to electrically distinguish the difference.
For this reason, there can be used a method of detecting the presence or absence of the transparent wafer with the aid of microswitches or the like. Since a contact type is employed in this case, the microswitches may possibly contaminate the wafers, or damage the peripheries of the wafers.